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Old 11-07-2010, 11:53 AM   #21 (permalink)
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Originally Posted by fusion210 View Post
Thanks for taking the time to do the test. You said that you used third gear when heavy, does that mean you didn't use third gear when you were light?
I usually use the highest gear possible without lugging the engine. I consider 1600 rpm to be the minimum. I used to use BSFC graphs, but found that in practice a slower turning engine and higher manifold pressure to be better. That is based on experience, and not science. :-)
For day to day driving I use 1st, 3rd, and 5th only.
For my A-B-A test I used 4th also.

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Old 11-11-2010, 10:00 PM   #22 (permalink)
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Seems like you would be adding wear to the main bearings of your engine by not using all of your gears when you accelerate to speed.


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I usually use the highest gear possible without lugging the engine. I consider 1600 rpm to be the minimum. I used to use BSFC graphs, but found that in practice a slower turning engine and higher manifold pressure to be better. That is based on experience, and not science. :-)
For day to day driving I use 1st, 3rd, and 5th only.
For my A-B-A test I used 4th also.
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Old 11-11-2010, 11:29 PM   #23 (permalink)
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Thanks for the write up.
Theoretically, if your on level ground, with predictable losses, starting from 55mph, with a 100%efficient engine and drive train; you should see no difference. The question then becomes: what is the engine doing. This is what could favor one weight over the other.
First off; the frequency of starting and killing the engine will slightly favor the lower frequency of the car with greater mass.
Second, the need for greater power to accelerate the higher mass car may effect eff of engine. Though, by expecting slower response and doing everything the same (gear/throttle position) this should be almost identical for both.
Third; aerodynamics, The higher mass car will sit lower to the ground. This may or may not be a good thing, but I would give the high mass car better odds on this one.
But in life; you must start from a stop, and there are hills, and people driving behind you. Acceleration is clearly bad with the high mass car, which will balance out gains that may have been possible.


As for the bullet comparison: I think people are missing some rather important points on this. If you have a identical lead bullet and a solid copper bullet with the same power behind them (in a theoretical "perfect gun") then they will both have the same power at the muzzle. The copper bullet will be going much faster. Does anyone here know anything about aerodynamics and speed? Really? So the faster bullet will have far more drag on it, taking away more energy, until it reaches the same speed as the lead one. But at that point it will have less energy because it has less mass, so it will continue to slow at a greater rate.
With the metro you have the same speeds, and different masses. So less powder is needed to get the copper bullet up to the same speed as the lead one. But the lead one will go further, as it has more energy. In a perfect world: It would take exactly the same amount of powder to send the lead or copper bullet one yard in this experiment.
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Old 11-11-2010, 11:41 PM   #24 (permalink)
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Thanks for the write up.
Theoretically, if your on level ground, with predictable losses, starting from 55mph, with a 100%efficient engine and drive train; you should see no difference.
Not true. In theory (and in practice) rolling resistance is proportional to vehicle weight. The question is whether the improvement in engine efficiency from easier P&G outweighs the increased rolling resistance. For most, the answer is no.
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Old 11-12-2010, 01:15 AM   #25 (permalink)
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Not true. In theory (and in practice) rolling resistance is proportional to vehicle weight. The question is whether the improvement in engine efficiency from easier P&G outweighs the increased rolling resistance. For most, the answer is no.
oh yeah, forgot that one. The tire patch will be proportionally larger on the higher mass car. And there is also mechanical rolling resistance, and this is one that could go either way. It is far more likely to favor the low mass car (less weight on the bearings) but depending on the suspension more mass could make the universal connector have to bend less and reduce resistance there (or make it worse).
There are really too many variables to accurately predict any outcome of this, but as always the key variable is the driver. It take time to get good at P&G with a new car, and that's basically what the high mass mod is. On this point, if you can get the same mpg right from the start, would a few months of practice give you better mileage? there is really only one way to find out.
If we look at it in the real world I think the differences between the high mass and low mass car will be minute, with the exception of getting up to speed. A little city driving could really kill a tank's number.
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Old 11-12-2010, 01:36 PM   #26 (permalink)
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Varn - As long as you're not revving overly high or lugging the engine too low, it has no effect on the engine. In some vehicle, the tranny ratios are spaced closer than they need to be, to optimize for heavy loads, or fast acceleration. When neither is needed, some gears can be skipped without any issue.
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Old 11-12-2010, 05:43 PM   #27 (permalink)
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Yeah you are right, it will work for a while but why not use the entire range. Shifts can be made at even lower rpm by not skipping. I bought one of the first new Toyotas with a 5 speed (73). To turn it in to a 3 speed just doesn't make sense.

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Varn - As long as you're not revving overly high or lugging the engine too low, it has no effect on the engine. In some vehicle, the tranny ratios are spaced closer than they need to be, to optimize for heavy loads, or fast acceleration. When neither is needed, some gears can be skipped without any issue.
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Old 11-12-2010, 11:51 PM   #28 (permalink)
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It seems to me that, all other things being equal (aerodynamics, etc.) except for weight, the simple fact is it takes more work to move the extra weight. It takes more gas to do more work. All this stuff about extra glide distances doesn't come for free. It takes the extra work to get the extra glide. And since the real world isn't perfect, there's plenty of inefficiency in the engine working and the car gliding. Under some conditions, the glide side may show an advantage, but overall, day in and day out, it takes more work to move more weight, and the lighter car should get the better mpg.
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Old 11-13-2010, 02:35 AM   #29 (permalink)
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It seems to me that, all other things being equal (aerodynamics, etc.) except for weight, the simple fact is it takes more work to move the extra weight. It takes more gas to do more work. All this stuff about extra glide distances doesn't come for free. It takes the extra work to get the extra glide. And since the real world isn't perfect, there's plenty of inefficiency in the engine working and the car gliding. Under some conditions, the glide side may show an advantage, but overall, day in and day out, it takes more work to move more weight, and the lighter car should get the better mpg.
This is for the most part quite true, but remember; with the exception of rolling resistance, it takes the same energy to keep a identical car of any mass moving at a constant speed. If you where to load your car with 800lbs of junk and inflated the tires proportionally (assuming they were not all ready at max pressure) you would get nearly the same mileage on a long highway trip. But if you had to stop and start many times in the trip you would suffer greatly.
With proper technique, for some people, more weight could work as an advantage for long trips at a relatively constant speed. This it not true for most people or situations, but may be for some. Notable elevation changes would add another consideration to this. I commuted down about 1000 feet over 12 miles every day. I was planning some water tanks, but I came in to a better solution; I moved in to the valley.
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Old 11-13-2010, 03:53 AM   #30 (permalink)
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Following is a terribly uncontrolled comparison, but I'll toss it up for illustration and discussion. I drove four of us to dinner 75 miles away, leaving from Bryce Canyon National Park at 8000' elevation and going to dinner in Boulder, UT. The extra 450 lbs of passengers in my 2200 lb. xB is a 20% weight increase. The drive has a final 1300' elevation drop to Boulder, but that doesn't tell the real story. There are 12 and 14% grades on the route that necessitated shifting to second gear to climb them. By the time I reached Boulder, I had a 50 mpg trip average per the Ultragauge. At the end of the roundtrip, back in the park, I'd averaged 42.7 mpg. I had to run the AC from time to time, and the lights on the entire trip back. I tried my usual P&G (the passengers were all interested in seeing how I got 50 mpg on the trip out).

Three days later, I repeated the trip, without the passengers, and didn't use the AC or lights, so it wasn't a very good comparison. Nonetheless, the results were much better in both directions without the load. I averaged 55 mpg to Boulder, and 47.5 mpg for the round-trip, about 5 mpg better in both directions. I averaged 140 mpg the first 7 miles to Tropic, UT, and an amazing 71.9 mpg for 48 mpg to Escalante, UT.

I probably could have improved my loaded mpg if I'd been able to concentrate better, didn't run the AC or lights, and learned to compensate for pulsing with the additional load, but I doubt I could have improved it 5 mpg in either direction. The extra load simply overwhelmed my 108 hp. The heavier load meant I simply couldn't accelerate at my usual 83 load. 83 on the Ultragauge had me barely maintaining speed, and I had to mash it into the 90s load range to accelerate on far too many of my pulses.

Interestingly, I still had to drop to second gear to climb the steepest 12% and 14% grades, but there were a lot of hills I climbed in 4th and 5th gear solo that had me a gear or two lower with the extra 450 lbs of passengers (one 230 lb male, one 90 lb female, one 120 lb. female, clothing).

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